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In situ analysis of optoelectronic properties of semiconductor nanostructures and defects in transmission electron microscopes

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					                                                                                          12

  In-Situ Analysis of Optoelectronic Properties of
   Semiconductor Nanostructures and Defects in
             Transmission Electron Microscopes
                                Yutaka Ohno1, Ichiro Yonenega1 and Seiji Takeda2
                                        1Institute   for Materials Research, Tohoku University
                                                                             2Osaka University

                                                                                        Japan


1. Introduction
A wide variety of optoelectronic devices such as photovoltaic (including solar cells and
photo detectors), photoemitting (including lasers) and photocatalytic devices have been
developed for more than five decades. The physical nature of such devices, especially
operated with visible and near-infrared light, is electronic transitions between pairs of
energy levels, typically in semiconductors. In addition to the conduction band and the
valence band, defect levels, i.e., localized energy levels associated with nanostructures and
lattice defects, are responsible for the electronic transitions. Various kinds of nanostructures
can be fabricated, spontaneously or artificially, inside and onto semiconductors. Also, lattice
defects, such as point defects (including vacancies, interstitials, dopant and impurity atoms)
and extended defects (including grain boundaries, stacking faults, dislocations, and point
defect clusters), can be introduced, inevitably or accidentally, during crystal growth and
device fabrication processes. Therefore, in order to fabricate optoelectronic devices with
advanced and ultimate functions, the structural properties of semiconductor nanostructures
and defects, as well as their optoelectronic properties such as the possible presence of defect
levels, should be understood with a high spatial resolution simultaneously with a high
spectral resolution.
Optical measurements such as luminescence and photoabsorption spectroscopy are
powerful techniques to determine defect levels. Since the spectral resolution is higher than
the order of 10−3 eV, this techniques are useful to study the energy levels in the low energy
range between the band edges of semiconductors (of the order of 100 eV at most), which
dominate the optoelectronic properties of the device products made of the materials.
Therefore, when the measurements are performed in a transmission electron microscope
(TEM), such as near-field optical measurements in a TEM (Ohno, 2010a), we can examine
the optoelectronic properties simultaneously with the structural properties in small regions
observed in-situ with the TEM.
With an extremely high spectral resolution in comparison with the other spectroscopic
techniques in TEM, such as energy dispersive x-ray spectroscopy (e.g., Terauchi et al, 2010)
and electron energy-loss spectroscopy (e.g., Kikkawa et al, 2007) with a resolution less than
about 10-1 eV, the optical measurements in TEM enable us to assess in detail defect levels in




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small regions. It is interesting to note that we can examine in-situ the optoelectronic and
structural properties of nanostructures and defects inside a material, which properties are
affected by the surrounding material and are not determined in-situ by the optical
measurements in scanning probe microscopes, such as scanning near-field optical
microscopy. As the continuing miniaturization and integration of optoelectronic devices, the
optical measurements in TEM have been established as an indispensable micro-
characterization technique. In this chapter, principles of the optical measurements in TEM,
i.e., cathodoluminescence (CL) spectroscopy and transmission electron microscopy under
light illumination are briefly summarized, and the resent analysis of some semiconductors
for optoelectronic devices are reviewed.

2. Principles of the optical measurements in TEM
2.1 CL spectroscopy in TEM
CL is a phenomenon of light emission induced by electron irradiation. CL light is emitted
from a region in which electrons are irradiated, and the optical parameters, such as the
photon energy, intensity and polarization, vary depending on the electronic structure in the
region. Therefore, CL spectroscopy performed in a TEM enables us to examine the electronic
structure simultaneously with the atomic structure in small regions observed in-situ with
the TEM, where electrons are irradiated. For example, the structural and compositional
variation, as well as the defect concentration and distribution, can be determined. Also, the
electronic properties such as defect levels and their carrier capture cross sections, which are
associated with the carrier lifetime and diffusion length, can be analyzed. In this subsection,
the principles underlying the generation and interpretation of CL signals are summarized.
The detailed descriptions of the principles including the pioneer work in 1978 (Petroff et al,
1978) are provided in a review (Yacobi and Holt, 1990).

2.1.1 Spatial resolution of CL measurements
Electrons irradiated into a material can undergo elastic and inelastic scattering. The
irradiated material is excited via inelastic electron scattering, and this excitation results in
the formation of x-rays, Auger electrons, secondary electrons, electron-hole pairs, and so
forth. CL lights are emitted via the recombination of electron-hole pairs. The spatial
resolution of CL measurements is, therefore, determined by the distribution of electron-hole
pairs.
When electrons are irradiated into a material, each electron changes its direction via an
elastic scattering, and it reduces its kinetic energy via an inelastic scattering. As a result of
the scattering processes, the original trajectories of the electrons are randomized. For a thin
solid material through which most incident electrons can transmit, used as a specimen in a
TEM, the shape of the electron penetration range (a so called generation volume) is conical
and the radius of a generation volume, which is the maximum at the electron exit surface, is
determined (Goldstein, 1979). Electrons and holes are generated inside a generation volume,
via some electron-electron interactions. They can diffuse in a material, and the distribution

density of minority carriers at a position r, Δn(r) obeys the differential equation of
of electron-hole pairs is dominated by the diffusion of minority carriers. The stationary

continuity, div[grad Δn(r)]D - Δn(r)/τ(r) + g(r) = 0, in which D is the diffusion constant for
minority carriers, τ is the mean recombination lifetime, and g is the generation rate of
electron-hole pairs. The distribution of minority carriers has been discussed theoretically




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and Defects in Transmission Electron Microscopes                                             243

(e.g., Everhart & Hoff, 1971, Donolato & Venturi, 1982), and the range in which minority
carriers exist is expected to be twice as large as a generation volume at most, for thin
materials. Therefore, the spatial resolution of CL measurements can be approximated to the
maximum diameter of a generation volume. The typical resolution is the order of 102 nm.

2.1.2 CL spectroscopy and analysis
CL lights are emitted via various radiative electronic transitions. One transition is the
recombination between an electron in the conduction band and a hole in the valence band (a
band-to-band transition), which is typical in direct gap semiconductors at high
temperatures. At a temperature T at which kT (k is the Boltzmann factor) is smaller than the
binding energy for free excitons, the excitonic level for the excitons is formed just below the
conduction band edge and the decay of the excitons (the free exciton transition between the
excitonic level and the valence band) results in a CL emission. When a defect exists, it may
induce donor and/or acceptor levels. Electrons and holes generated by electron irradiation
are trapped at the levels, and CL lights may be emitted via a donor-to-valence band
transition, a conduction band-to-acceptor transition, and a donor-to-acceptor transition.
They are the typical transitions in indirect gap semiconductors at high temperatures. Similar
to the case of free excitons, the excitonic levels bound to a donor and an acceptor are formed
at low temperatures and CL lights may be emitted via the decay of the excitons (bound
exciton transitions). Also, when an impurity with incomplete inner shells (such as a rare
earth ion or a transition metal) exists, its excitation and radiative deexcitation (an inner shell
transition) results in a CL emission. Detailed recombination processes are reviewed, for
example, by Henderson & Imbusch (Henderson & Imbusch, 1989) for inner shell transitions,

The photon energy of a CL light emitted via a transition is written as ΔE+δ, in which ΔE
and by Yu & Cardona (Yu & Cardona, 2001) for the other transitions.

corresponds to the energy difference between the energy levels concerning the transition. δ
is a positive value for a donor-to-acceptor transition (Yu & Cardona, 2001), and it is zero for
the other transitions. CL energy measurements have been used to assess energy levels in a
small volume, which are connected to the structure including defects and composition in the
volume.

radiative recombination of σ, exist in a small volume, the number of photons emitted from
When the number N of nanostructures or defects of the same kind, with the cross section for

the volume per a unit time is proportional to Nσ. CL intensity measurements have been
applied to assess the density of point defects acting as radiative recombination center (e.g.,
Graham et al, 1994, Mitsui et al, 1996) and those acting as non-radiative one (e.g., Ohno et al,
1999, Ohno & Takeda, 1996). Also, the techniques have been used to assess the carrier
dynamics such as excited carrier paths towards the lower energy states (e.g., Akiba et al,
2004, Merano et al, 2005, Sonderegger et al, 2006), as well as carrier lifetimes.
Some extended defects such as dislocations (Yamamoto et al, 1984, Mitsui & Yamamoto,
1997), twins (Ohno et al, 2007b), platelets (Ohno, 2005a) and strains induced by a uniaxial
stress (Wang et al, 1993, Ohno, 2005b), as well as low-dimensional nanostructures such as
nanowires (Ishikawa et al, 2008, Yamamoto et al, 2006), quantum wells (Ohno, 2005a) and
superlattices (Ohno & Takeda, 2002, Ohno et al, 2007b), form anisotropic defect levels, and
electronic transitions via the levels result in the emission of polarized CL lights. CL
polarization analyses have helped us to assess quantitatively the atomistic structures of such
nanostructures and defects.




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2.1.3 CL imaging and analysis
A CL image (or a CL intensity map) is obtained by detecting the CL intensity as an electron
beam scans a raster over a specimen. When there is a microstructure, such as a defect, in
which the cross section for radiative or non-radiative recombination differs from that in the
surrounding homogeneous material, contrast arises in CL images. For example, the spatial
distribution of defects in degraded devices (e.g., Sieber et al, 1993, Cheng et al, 1995, Bonard
et al, 1998) have been determined.
CL contrast due to a defect varies depending on the atomistic structure, size and location of
the defect. Among them, effects of dislocations on CL contrast have been determined
theoretically (e.g., Jakubowicz, 1986, Schreiber & Hildebrandt, 1994, Holt & Napchan, 1994),
and the radiative and non-radiative lifetime and diffusion length for minority carriers
around a dislocation have been assessed.

2.2 Electron microscopy under light illumination
CL spectroscopy in TEM is a useful technique to determine defect levels of nanostructures
and defects acting as radiative recombination center. However, a defect level acting as non-
radiative recombination center is, in general, difficult to assess. Also, it is difficult to
examine the process of an electronic excitation (i.e., an electronic transition from an energy
level to a higher one) via a defect level, since various kinds of electronic excitaitions take
place simultaneously during the irradiation of electrons whose energy is much higher than
the band gap energy. In order to determine such electronic transitions with a high spatial
resolution, transmission electron microscopy under light illumination has been used, since
the pioneer work in 1984 (Suzuki et al, 1984). In this subsection, the principles underlying
the electronic transitions induced by light illumination are summarized.

2.2.1 Electronic transitions due to light illumination
Unlike electronic transitions due to electron irradiation, induced via an interaction between
electrons, electronic transitions due to the illumination of a light, especially in visible and
near-infrared region, are mainly induced via an interction of photons with electrons. For a
photo-induced electronic transition between two energy levels, the transition rate is high
when the photon energy of the illuminating light is close to the energy difference between
the levels (in a resonant photo-excitation condition), according to the Fermi’s golden rule.
Therefore, by choosing a proper photon energy which corresponds to the energy difference
between a defect level and an another level, we can selectively determine the defect level.
When the technique is applied in TEM, it is useful to determine the nanostructures and
defects whose atomistic structures vary under photo-excitation conditions.
Under the illumination of a light with a proper photon energy, electron-hole pairs can be
generated. Therefore, the spatial resolution of optical measurements with photo-excitation
is approximated to the diameter of the generation volume, similar to CL measurements.
Since a light beam almost goes straight inside a thin material, the resolution is
approximated to the diameter of the beam. In case of the illumination of a far-field light in
TEM with a conventional optical system, such as an optical fiber (Suzuki et al, 1984,
Yoshida et al, 2004), optical lens (Ohno & Takeda, 1995), and parabolic mirror (Ohno,
2010a), the resolution is much less than 103 nm due to the diffraction limit of light. The
spatial resolution beyond the limit, higher than 102 nm, is acheived in a TEM with a near-
field light probe (Ohno, 2010a).




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In-Situ Analysis of Optoelectronic Properties of Semiconductor Nanostructures
and Defects in Transmission Electron Microscopes                                             245

2.2.2 In-situ photo-excitation in TEM and analysis
Transmission electron microscopy under light illumination has been used to study in-situ
photo-induced structural variations such as the dislocation motion related to photo-plasticity
(Suzuki et al, 1984) and degradations in photoemitters (Ohno, 2005b, 2010a, Ohno et al, 2009b);
the glide of dislocations under light illumination is observabed in-situ. Photocatalytic reactions
in catalysts are also observable in-situ, and a photo-induced decomposition (Yoshida et al,
2004), structural transformation (Yoshida et al, 2006), and nucleation of nanostructures
(Yoshida et al, 2007) are observed in a photocatalytic system. Such a structural variation is
induced via a photo-induced electronic excitation, and depends on the photon energy,
intensity and polarization of the illuminating light. A defect level related to the photo-
excitation can be estimated with a proper photon energy, since the degree of the structural
variation is the maximum when the photon energy is close to the energy difference between
the defect level and an another level (Ohno, 2005b, Ohno et al, 2009b, Ohno, 2010a).
Semiconductors under light illumination can emit photoluminescence (PL) light, similar to
CL, so as to release some of the photon energy absorbed from the illuminating light. PL
spectroscopy has an advantage over CL spectroscopy when various kinds of nanostructures
and defects coexist in a small region. A defect level of nanostructures or defects of the same
kind can be determined selectively, since the emission efficiency is increased in a resonant
photo-excitation condition in which the illuminating light is absorbed via the defect level.
Also, Raman spectroscopy, by which the phonon structure of nanostructures and defects can
be determined, is available under light illumination, in a resonant photo-excitation
condition. PL and Raman spectroscopy with a far-field light is demonstrated in TEM (Ohno
& Takeda, 1995, 1996, Ozaki et al, 1998, Ohno et al, 1999, Kohno et al, 2004, Ohno, 2010a).

2.2.3 In-situ photo-excitation systems in TEM
As a convenient system, an optical fiber is installed inside a TEM so that the end of the fiber
is located near the area observed with the TEM (Suzuki et al, 1984, Yoshida et al, 2004). Even
though the system is easily constructed in any TEMs, the spatial resolution is rather low;
almost all areas on a TEM specimen are illuminated. Here a high resolution system for in-
situ photo excitation is reviewed.
Figure 1 shows an apparatus for in-situ high-resolution optical measurements with a near-
field light probe in a TEM (Ohno, 2010a). A metal tip is located close to the area observed
with the TEM. The tip is then illuminated with a far-field light, and an intense near-field
light is induced in the immediate vicinity of the tip apex, due to a surface enhanced effect
(Inouye & Kawata, 1994).
In detail, a specimen in a TEM is illuminated with a laser light by using a parabolic mirror.
The mirror is truncated so that it is inserted inside a small space in the TEM and the
specimen is illuminated with an electron beam for TEM observation. A laser light is
introduced into the TEM so that the optical pass is parallel to the rotation axis of the
parabolic mirror, and the introduced light is focused onto the focal point of the mirror. With
a CL technique (Ohno & Takeda, 1995), the focal point is set onto the area observed with the
TEM. A specimen generally exhibits CL emission during TEM observation, and the CL light
emitted from the observed area is detectable optically. Also, a laser light focused on a
specimen is detectable. Each light is observed as a bright spot with an optical zoom
microscope. By adjusting the location of the parabolic mirror so that the laser spot coincides
to the CL spot, the laser light is precisely focused onto the area observed with the TEM. In
order to form a near-field light, a metal tip is located on the observed area with a TEM-STM




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246                                                          Optoelectronic Devices and Properties

holder (Figs. 1c and 1d). The tip is set in the side illumination configuration in which the tip
axis is about normal to the optical pass of a laser light for inducing a near-field light. With
the configuration, an intense near-field light can be induced at the tip apex, due to a p-
polarization effect (Hayazawa et al, 2002).




Fig. 1. (a) Schematic and (b) external views of an apparatus for in-situ near-field optical
measurements in a TEM. (c) Optical and (d) TEM images of a specimen and a metal tip in a
TEM-STM holder

3. Optoelectronic properties of semiconductor nanostructures and defects
3.1 Dislocations in wide gap semiconductors
Extended defects in semiconductors frequently affect the optoelectronic properties. Among
them, dislocations are key topics for developing the relevant devices fabricated with wide
gap semiconductors, such as GaN and ZnO, since they are inevitably introduced a high
density of dislocations. In this subsection, the recent knowledge of the optoelectronic
properties of dislocations, mainly revealed by in-situ optical measurements in a TEM, is
reviewed. The properties in GaN, which is the pioneer of the commercial photoemitting
device operated in the short wavelength range, are first summarized, and those in ZnO,
which is attracting keen interest as a promising analog to GaN, are shown.
GaN and related materials have been applicated to short-wavelength optoelectronic devices
since the successful fabrication of long-lifetime blue photoemitting diodes operated at room
temperature (Nakamura et al, 1994). Due to the lack of a suitable growth substrate, there
exist mismatches in lattice constant and in thermal expansion coefficient between a GaN
epilayer and its substrate (e.g., sapphire or SiC), and the mismatches induce a high density
(106-1010 cm-2) of dislocations. However, the epilayer has a high efficiency of light emission.




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It is well known that most dislocations act as non-radiative recombination center in III-V
(e.g., Petroff et al, 1980, Hutchinson et al, 1984, Wang et al, 1992) and II-VI (e.g., Mitsui &
Yamamoto, 1997, Mitsui et al, 1996) compound semiconductors, and the dislocations affect
the device lifetimes even when the density is as low as 104 cm-2. Therefore, the optical
properties of dislocations in GaN have been a subject of controversy. Individual
dislocations, grown-in (Yamamoto et al, 2003, Nakaji et al, 2005, Albrecht et al, 2008) and
introduced intentionally (Albrecht et al, 2002, 2008), are examined in TEM, and the
dislocations act as recombination-active centers. Actually, they often exhibit recombination-
enhanced glide (Maeda et al, 1999). The dislocations lying on a basal plane (i.e., misfit
dislocations) seem to be undissociated with the Burgers vector b of 1/3<1120> (Albrecht et
al, 2002), even though they can be dissociated (Suzuki et al, 1994), presumably due to a high
energy barrier for an undissociated dislocation to dissociate (Blumenau et al, 2003). Among
the dislocations, 60o dislocations are proposed to act as radiative recombination center that
emit a light with photon energy of 2.9 eV at low temperatures, while the other dislocations
act as non-radiative recombination center (Albrecht et al, 2002). The screw dislocations have
a small recombination activity in comparison with the edge dislocations (Yamamoto et al,
2003), even though their velocity for recombination-enhanced glide is faster in comparison
with the edge dislocations (Maeda et al, 1999). Also, threading dislocations, i.e., dislocations
lying on pyramidal planes with b = 1/3<1120> (Albrecht et al, 2008) and those on prismatic
planes with b = 1/3<1120> or 1/3<1121> (Yamamoto et al, 2003), act as non-radiative
recombination center. Even though the threading dislocations have a smaller recombination
activity in comparison with the misfit dislocations (Yamamoto et al, 2003), they reduce
overall luminescence intensities (Yonenaga et al, 2006). The diffusion length of minority
carriers around a dislocation is estimated (Yamamoto et al, 2003, Nakaji et al, 2005, Ino &
Yamamoto, 2008), and it is smaller in comparison with the other semiconductors; e.g., about
200 nm, 150 nm, and 60 nm, respectively, for non-doped, Mg-doped (1x1017 cm-3), and Si-
doped (3x1018 cm-3) GaN, even at low temperatures. The estimated length is smaller than the
nearest-neighbor distances of dislocations, and this would be a reason why GaN can tolerate
a much higher dislocation density than in the other semiconductors.
Wide gap II-VI compound semiconductors had been expected to be the most promising
materials for short-wavelength optoelectronic devices, but most researches were closed after
the commercialization of GaN-based devices. However, there is a revival of interest in the
materials because of their potential applications in optoelectronic devices employing
excitonic effects in the short wavelength range, due to their large exciton binding energy,
etc. Among them, ZnO has rapidly emerged as a promising analog to GaN, after the
demonstration of the excitonic emission at elevated temperatures up to 550 K (Bagnall et al,
1998). Despite considerable success in optimizing the growth conditions and structural
quality such as the fabrication of blue photoemitters (Tsukazaki et al, 2005), ZnO epilayers
still contain a number of defects. The most characteristic defect, investigated with a TEM, is
a high density (109−1011 cm-2) of dislocations (e.g., Setiawan et al, 2004). In ZnO, dislocations
are introduced with lower stress and are much mobile, in comparison with GaN (Yonenaga
et al, 2008). The knowledge of the influence of dislocations is, therefore, required for the
practical use of this material.
In order to determine the effects of dislocations on the optoelectronic properties, an arbitrary
number (109-1010 cm-2) of dislocations on a basal plane are introduced in wurtzite ZnO
single-crystals, at elevated temperatures (923-1123 K) comparable to the typical
temperatures for the fabrication of ZnO-based devices, by compressive deformation (Ohno




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et al, 2008a). The dislocations seem to be undissociated with b = a/3<1120>, even though
they can be dissociated (Suzuki et al, 1994), similar to wurtzite GaN (Suzuki & Takeuchi,
1999, Albrecht et al, 2002). Most dislocations are of mixed type including 60o dislocations,
and screw and edge dislocations also exist. Macroscopic PL measurements reveal that,
unlike wurtzite GaN, the introduction of the dislocations do not influence all the emission
bands existing in as-grown specimens (Fig. 2a). Also, additional emission bands, with
photon energies EL of 3.100 and 3.345 eV at temperature of 11 K, as well as their LO phonon
replicas (with the separation between the nearest-neighbor emission lines of 72 meV), are

defect level ΔE associated with the 3.100 eV emission and that with the 3.345 eV one are
introduced (Fig. 2b). From the analysis of the thermal quenching processes, the depth of the

estimated to be 0.3 eV and 0.05 eV, respectively. Since the PL intensities increase with
increasing dislocation density, the emission bands are associated with the dislocations
introduced at elevated temperatures (Ohno et al, 2008b).




Fig. 2. PL intensity vs photon energy for a specimen deformed at 923 K (the red curve) or
annealed at 923 K without stress (the blue curve). The ratio of those PL intensities,
R=Ideformed/Iannealed, is also shown (the green curve). A part of (a) is magnified in (b). The inset
in (a) shows a TEM image of dislocations in the deformed specimen. (Ohno et al, 2008b)
In-situ TEM observation with 80 keV electrons reveals that (Ohno et al, 2009b), screw and
edge dislocations glide forming mixed dislocation segments when electrons are irradiated at
temperature of 120 K, as well as at room temperature (Fig. 3). Effects of the point defects
introduced by the irradiation is negligible, since the electron energy is much smaller than
the threshold electron energy for displacement damage in ZnO (310 keV). The glide velocity
for a dislocation is independent of irradiation temperature, and it increases with increasing
electron flux. Therefore, the dislocations glide via an electron-hole recombination (Maeda et
al, 2000), and the screw and edge dislocations act as non-radiative recombination center. The
glide velocity for a screw dislocation is more than 101 times larger in comparison with an
edge dislocation. Those properties are similar to wurtzite GaN (Maeda et al, 1999).
In order to determine the optoelectronic properties of each individual dislocation, the
recombination-enhanced glide is determined in detail by TEM observation under the
illumination of a light, in the energy range of 2.25-2.92 eV, at temperature of 120K (Ohno et
al, 2009b). A screw dislocation glides when a light with photon energy around 2.5 eV is
illuminated (Fig. 4). Since the estimated energy is smaller than the band gap energy Eg
(about 3.4 eV at 120 K) and is larger than the activation energy of about 1 eV for thermal
dislocation glide (Yonenaga et al, 2008), electrons and holes would be recombined at a defect

(Ohno, 2005b). Namely, the depth of the defect level ΔE is about 0.9 eV (Eg–2.5 eV). Since the
level associated with the dislocation, similar to the case of 90◦α partial dislocations in ZnSe




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recombination-enhanced glide of edge dislocations is not observed, their defect level would
locate deeper than 1.15 eV (Eg–2.25 eV) and they might glide under the illumination of a
light with the corresponding photon energy.




Fig. 3. Dislocation glide due to electron irradiation. The specimen is irradiated with
electrons with flux of 5x1016 e mm-2 s-1 at room temperature for 3600 s (a to b). It is
subsequently irradiated with flux of 5x1017 e mm-2 s-1 at room temperature for 900 s (b to c),
then irradiated with the same flux at temperature of 120 K for 900 s (c to d). The black curves
in b-d indicate the location of a screw dislocation before electron irradiation. (e) A schematic
view of the dislocations with b = a/3[1210] (green curves), a/3[1120] (blue curves), and
a/3[2110] (red curves) in (a). (Ohno et al, 2009b). Copyright Wiley-VCH Verlag GmbH &
Co. KGaA. Reproduced with permission




Fig. 4. (a – f) Dislocation glide due to light illumination. Dislocations are illuminated in turn
with a light with the photon energy Eil. (g) A schematic view of the dislocations with b =
a/3[1210] (green curves), a/3[1120] (blue curves), and a/3[2110] (red curves) in (a). (h)
Variation of the line shape of the screw dislocation in the squared area in (a) during light
illumination. (Ohno et al, 2009b). Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission




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250                                                            Optoelectronic Devices and Properties

The emission band peaking at 3.1 eV, due to a defect level of 0.3 eV depth associated with
dislocations (Ohno et al, 2008b), is determined by CL spectroscopy (Ohno et al, 2009b). Since a
CL light emitted from a dislocation is rather weak, CL lights are obtained from an area in
which a number of dislocations exist (the total length of the dislocation lines of about 2x103
nm). The lights are collected after the light illumination with the same illumination sequence
as in Fig. 4, and the intensity is unchanged during the illumination (Fig. 5). The intensity is also
unchanged during electron irradiation (Fig. 6). Therefore, the emission is not influenced by the
conversion of screw and edge dislocations into mixed dislocations, via their recombination-
enhanced glide under light illumination or electron irradiation. Those results indicate that the
origin of the emission band is neither the screw dislocations nor the edge ones.




Fig. 5. (a) CL spectra after the illumination of a light with the photon energy Eil as in Fig. 4.
(b) The CL intensity at 3.12, 2.43, or 2.18 eV vs Eil. (Ohno et al, 2009b). Copyright Wiley-VCH
Verlag GmbH & Co. KGaA. Reproduced with permission




Fig. 6. (a) CL spectra after the irradiation of electrons, with flux of 5x1017 e mm-2 s-1 at
temperature of 120 K, for the irradiation time tir. (b) The CL intensity at 3.12, 2.43, or 2.18 eV
vs tir. (Ohno et al, 2009b). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced
with permission
It is theoretically expected that a 60o dislocation in wurtzite GaN, which is similar to ZnO in
the band gap energy, form a defect level 0.3 eV (~0.09Eg) above the valence band edge
(Blumenau et al, 2003), and the emission due to the corresponding level is proposed
(Albrecht et al, 2002). Similar emission bands related to dislocations with photon energy EL
are also observed in a wurtzite II-VI compound of CdS (Negrii & Osipyan, 1978) and
another compounds with the zinc blende structure such as ZnSe (Hilpert et al, 2000), ZnTe
(Naumov et al, 1993), CdTe (Shreiber et al, 1999) and GaAs (Fravacque et al, 1989), even

bands is 90o dislocations with a defect level of ΔE ~ 10-1Eg (Table 1). Those results suggest
though the dislocations are dissociated, and it is proposed that the origin of the emission




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that a 60o complete dislocation in ZnO is dissociated into a pair of a 90o partial dislocation
and a 30o one, with a narrow separation which is difficult to observe with a conventional
TEM technique, and the 90o partial dislocation would induce the 3.1 eV emission.

              structure      Eg [eV]     EL [eV]    ΔE [eV]      ΔE/Eg            model
    GaN       wurtzite         3.5         2.9         0.3       0.086      60o or 90o partials
    ZnO       wurtzite        3.44        3.100        0.3       0.087          90o partials
                                                                            with point defects?
    ZnSe     zinc blende       2.82        2.61        0.22       0.078      Se(g) 90o partials
    CdS        wurtzite        2.58       2.430       0.153       0.059              ?
                                          2.439       0.144       0.056
                                          2.447       0.136       0.053
    ZnTe     zinc blende       2.39       2.185        0.21       0.088         Te(g) 90o partials?
                                          2.148        0.24       0.100
   CdTe      zinc blende       1.61        1.48       0.13        0.081         Te(g) 90o partials
   GaAs      zinc blende       1.52        1.13       <0.2       <0.132         As(g) 90o partials
Table 1. Emission bands related to dislocations
Figure 7 summarizes the atomistic structure of dislocations on a basal plane introduced at
elevated temperatures, proposed in this ZnO work. 90o partial dislocations in the dissociated
60o dislocations would form a defect level of 0.3 eV depth acting as radiative recombination
center, while the screw and edge dislocations form a defect level near the mid gap acting as
non-radiative recombination center, similar to wurtzite GaN (Albrecht et al, 2002).




Fig. 7. A schematic view of the defect levels of misfit dislocations in ZnO
Misfit dislocations in ZnO introduced at elevated temperatures do not influence the
emissions except the dislocation-related emissions, as in Fig. 2a. On the other hand, almost
all emissions in ZnO are suppressed due to the introduction of misfit dislocations at room
temperature (Coleman et al, 2006, Takkouk et al, 2005). Since those dislocations are similar
in type, the atomistic structure of a dislocation would be modified at elevated temperatures,
presumably due to an interaction of the dislocation with point defects. Indeed, a dislocation
involving point defects is the candidate for the 3.1 eV emission band (Ohno et al, 2008b).
Also, the intensity of the green (2.43 eV) and yellow (2.18 eV) emissions, associated with O-
and Zn-vacancies (Zhao et al, 2005), varies when dislocations modify their structure via their
glide (Figs. 5 and 6). This characteristic of ZnO may be an advantage over GaN, since all
emissions in GaN are suppressed when dislocations are introduced even at elevated
temperatures (Yonenaga et al, 2006), as well as at room temperature. (Kucheyev et al, 2000).
The interaction of dislocations with point defects is a key to elucidate the influence of
dislocations on the optoelectronic properties in ZnO. In order to understand the properties




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252                                                              Optoelectronic Devices and Properties

at an atomistic level, it is needed to determine an individual dislocation with a higher spatial
resolution, as well as with a high spectral resolution. Near-field optical measurements in a
TEM (Fig. 8) will be a unique technique to reveal the subject.




Fig. 8. A preliminary result for in-situ examination of a photo-induced glide of an individual
dislocation in ZnO. TEM images of the screw dislocation indicated with the arrowhead in
(a); before (a), under (b), and after (c) the illumination of a near-field light with photon
energy of 2.41 eV. The broken curve in (c) indicates the dislocation line before the
illumination. (Ohno, 2010a)
It is shown that transmission electron microscopy under light illumination is available to
determine quantitatively the defect levels acting as non-radiative, as well as radiative,
recombination centers, with a high spatial resolution. By means of this technique, combined
with another in-situ techniques such as environmental TEMs (e.g., Yoshida et al, 2008, 2009)
and in-situ electrifying (e.g., Nogami et al, 2009, Kohno et al, 2009), the dynamic properties
of nanostructures and defects under electronic excitation will be determined.

3.2 Quantum nanostructures in middle gap semiconductors
After the successful fabrication of quantum wells based on middle gap III-V compound
semiconductors including AlGaAs, around 1970, various kinds of quantum nanostructures
have been fabricated. For example, by using the differences in the surface migration of
adatoms and using the Stranski-Krastanov growth, quantum wires and quantum dots have
been self-organized. Also, by means of modern crystal-growth techniques such as metal-
organic chemical-vapor deposition and molecular-beam epitaxy, superlattices based on
periodic changes of either material compositions or doping patterns have been grown. Such
quantum nanostructures have potentials for enhancing the device performance, and also
their electron confinement produces quantum electronic states which provide an important
system for fundamental physics.
Recently, superlattices based on periodic changes of crystal directions, i.e., twinning
superlattices (Ikonic et al, 1993), have been proposed in III–V (Xiong & Eklund, 2006, Ohno et
al, 2007b, Algra et al, 2008, Bao et al, 2008), II-VI (Ikonic et al, 1996), and VI (Hibino et al, 1998,
Fissel et al, 2006) semiconductors and in metals (e.g., Kobayashi & Uchihashi, 2010). It is
expected that they can offer as much versatility in tailoring the miniband structure as there
exists in heterostructure-based superlattices (e.g., Ikonic et al, 1993, Nakamura & Natori, 2006).
In this subsection, the optoelectronic properties of twinning nanostructures self-organized in
indirect gap Al0.5Ga0.5As epilayers, revealed by CL spectroscopy in a TEM, are reviewed.
An Al0.5Ga0.5As epilayer without extended defects is traditionally grown on a substrate that
is flat and electrically neutral; the substrate is oxidized intentionally, to form a flat interface
between the oxide and the substrate, and the oxide is then removed by the annealing in an
As atmosphere, for fear of the sublimation of As atoms on the substrate. On the other hand,
when a substrate is not treated the pretreatments, multiple twin boundaries are induced on




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surface (Ohno et al, 2008c). Twin boundaries of Σ3 type are formed on (111) and (111) (Fig.
the substrate; the boundaries are induced in an epilayer grown on a rough As-deficient

9). No compositional fluctuation is detected around the boundaries, indicating that
heterostructures, such as AlxGa1-xAs/AlyGa1-yAs superlattices and impurity agglomerates,
are not self-organized around the boundaries.




Fig. 9. Diffraction patterns of a twinned Al0.5Ga0.5As epilayer taking along (a) [110], (b) [011],
and (c) nearly [011]. TEM images taking with a twin spot at (d) 1/3, 1/3, 5/3 and (e) 1/3,
1/3, 5/3. (f) A high-resolution TEM image of twin boundaries. (Ohno et al, 2007b)




Fig. 10. (a) The experimental setup for polarized CL spectroscopy. (b) A panchromatic CL
intensity map and (c) the corresponding bright-field TEM image of a twinned Al0.5Ga0.5As
epilayer grown on an Al0.2Ga0.8As. (d)-(g) Monochromatic CL intensity maps, and (h)-(i) the

with the solid lines. The photon energy of the CL light is; (d, e) 1.82 eV or (f, g) 1.85 eV. φ =
corresponding dark-field TEM images in which the locations of boundaries are indicated

(d, f) 0o or (e, g) 90o. (h) or (i) is, respectively, taken with a twin spot of α or β in (j). (k)
Polarized CL spectra for φ = 0o and 90o from the square area of Fig. (h). (Ohno et al, 2007b)




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CL mapping measurements reveal that (Ohno et al, 2007b), a CL light is emitted from a
twinned layer, and the intensity is stronger in comparison with the band-to-band emission
in direct-gap Al0.2Ga0.8As (Figs. 10b-10c), even though the layer is an indirect gap
semiconductor. A CL light is emitted from an area in which many parallel boundaries are
arranged at similar intervals of nanometer length, and no CL light is detected in the other
areas (Figs. 10d-10i).
The distribution of the boundaries in a light source is characterized with the intervals
between the nearest-neighbor boundaries and the distribution function of boundaries g(l)
(Ohno, 2010b). g(l) is defined as the number of boundaries in the range of l to l+Δl with the
origin at a boundary divided by the number of boundaries N, similar to a distribution
function for an extended defect (Ohno et al, 2007a): g(l) = 1/N ΣNi=1n(l)/Δl in which n(l) is
the number of boundaries in the range of l to l+Δl for the i-th reference boundary, l is the

indicated with α in Fig. 11b. There is no translational symmetry in the arrangement of the
distance from the i-th reference boundary. Figure 11 shows an analysis of a light source,

boundaries, even though the intervals are much the same (Fig. 11c). However, they do not
distribute randomly but orderly in a short range. There are three peaks arranged at similar
intervals (peaking at the intervals of 2–5, 7–9 and 12–14 nm) in the distribution function for
the (111) boundaries (Fig. 11e). Therefore, four or more (111) boundaries are arranged with
the period of about 4 nm. Similar results are obtained at the areas from which a
monochromatic CL light is emitted.




Fig. 11. (a) CL spectra obtained from the encircled areas α and β in (b) [the insets in (b) show

the intervals between the nearest-neighbor boundaries in α or β. (e) or (f), respectively,
the locations of the boundaries in the areas]. (c) or (d), respectively, shows the histogram of

shows the distribution function of boundaries in α or β. (Ohno, 2010b)
The intensity profile of a CL light emitted from a set of parallel boundaries vs the photon
energy is fitted with a Lorentz function (e,g., Fig. 11a), and the half-width at half-maximum
(5–6 meV) is narrower in comparison with the band-to-band emission in direct-gap
Al0.2Ga0.8As (9–10 meV) and with an impurity emission (about 20 meV). The CL light is
polarized parallel to the boundaries (Figs. 10d-10i, Fig. 12), and the photon energy increases
with decreasing the interval of the boundaries. Those results indicate that a light source is a
set of parallel twin boundaries ordered in a short range.
By means of temperature T-dependent CL spectroscopy (Ohno, 2010b), the energy level
induced by ordered boundaries is determined (Fig. 13). The intensity of a CL light with the
photon energy of EL, I can be fitted with a function: I(T) = I0/[1 + Cexp(ΔE/kT)] where ΔE is




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the activation energy for the thermal quenching process and I0 or C is a constant (Holtz et al,
1985). ΔE for a CL light due to ordered boundaries is higher in comparison with bound
excitons, and it increases with decreasing EL. The estimated energy is close to the difference
between the band gap energy Eg and EL, indicating that the light is emitted via an electronic
transition between an energy band and a defect level associated with the boundaries;
thermal escape of the carriers trapped in the level is responsible for the quenching.




Fig. 12. (a) Polarized CL spectra for different φ obtained from α in Fig. 11b. The experimental

[001]. The CL intensity I vs φ; the z axis is parallel to (b) [001] or (c) [110] (Ohno et al, 2007b)
setup is similar to that in Fig. 10a, and the specimen is set so that the z axis is parallel to




intensity for the emission line α and that for β in (a). (Ohno, 2010b)
Fig. 13. (a) CL spectra obtained at various temperatures T. (b) T-dependence of the CL

Electron flux-dependent CL spectroscopy reveals that (Ohno et al, 2007c), the photon energy

intensity obeys a power law (Schmidt et al, 1992), and the power α is about 2.0. These results
of a CL light due to a set of ordered boundaries is independent of electron flux. The


donor–acceptor pair recombination (α < 1) and an excitonic transition (1 < α < 2).
indicate that the CL light is emitted via a band-to-band electronic transition, rather than a

Figure 14 shows the energy diagram based on a twinning superlattice theory (Ikonic et al,
1993). Suppose a twinning superlattice structure, which forms a narrow miniband whose
energy is lower than the energy at the conduction band edge, is embedded in the indirect
gap Al0.5Ga0.5As crystal. According to the model, an intense monochromatic CL light with
photon energy EL, polarized parallel to the boundaries, is emitted via the direct transition
between the miniband and the valence band. The thermal escape of electrons from the
miniband into the conduction band in the adjacent Al0.5Ga0.5As crystal is responsible for the
quenching of the CL emission, and the activation energy for the quenching ΔE is Eg−EL.
It is shown that CL spectroscopy in a TEM is available for the quantitative analyses of the
optoelectronic and structural properties of nanostructures with a high spatial resolution
(higher than about 80 nm as in Fig. 10d). The technique will be applied to explore novel




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256                                                          Optoelectronic Devices and Properties

nanostructures and defects with useful optoelectronic functions, as well as to assess the
functions at an atomistic level, inside semiconductors.




Fig. 14. A schematic view of band-edge energies for an ordered boundaries embedded in an
Al0.5Ga0.5As layer in which no twin boundary exists

3.3 Silicon and related materials as optoelectronic materials
Si and related materials are still the most important semiconductor for various kinds of
photovoltaic and electronic devices. However, small numbers of the optical assessments
have been performed in TEM since Si is an indirect-gap semiconductor. Even though 60o
and Lomer-Cottrell dislocations are believed to act as radiative recombination center (Higgs
et al, 1992, Sekiguchi & Sumino, 1996), they have not been determined in TEM. Some
preliminary optical measurements for Si nanostructures which emit visible light, such as
porous-Si (Itoh et al, 1996) and Si nanowires (Ozaki et al, 1998, 2005, Kohno et al, 2004), have
been performed in a TEM. With a recent progress in optical measurement systems used in
near-infrared region, optoelectronic properties in Si including nanostructures and extended
defects, which would be modified by the interactions with point defects (e.g., Inoue et al,
2008, 2009, Ohno et al, 2009a, 2010), will be elucidated at an atomistic level.

4. Concluding remarks
In-situ optical measurements in TEM are powerful and unique techniques to examine the
atomistic structures of small regions inside materials, with an extremely high spectral
resolution. This in-situ technique in TEM will be applied permanently to discover and assess
various novel nanomaterials.

5. Acknowledgements
This work was partially supported by the Ministry of Education, Culture, Sports, Science,
and Technology, Japan, Grant-in-Aid for Scientific Research (B) #19310072 (2007–2009) and
for Scientific Research on Priority Areas #21016002 (2009-2010), and the Nano-Materials
Functionality Creation Research Project in IMR, Tohoku University.

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                                      Optoelectronic Devices and Properties
                                      Edited by Prof. Oleg Sergiyenko




                                      ISBN 978-953-307-204-3
                                      Hard cover, 660 pages
                                      Publisher InTech
                                      Published online 19, April, 2011
                                      Published in print edition April, 2011


Optoelectronic devices impact many areas of society, from simple household appliances and multimedia
systems to communications, computing, spatial scanning, optical monitoring, 3D measurements and medical
instruments. This is the most complete book about optoelectromechanic systems and semiconductor
optoelectronic devices; it provides an accessible, well-organized overview of optoelectronic devices and
properties that emphasizes basic principles.



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Yutaka Ohno, Ichiro Yonenega and Seiji Takeda (2011). In-Situ Analysis of Optoelectronic Properties of
Semiconductor Nanostructures and Defects in Transmission Electron Microscopes, Optoelectronic Devices
and Properties, Prof. Oleg Sergiyenko (Ed.), ISBN: 978-953-307-204-3, InTech, Available from:
http://www.intechopen.com/books/optoelectronic-devices-and-properties/in-situ-analysis-of-optoelectronic-
properties-of-semiconductor-nanostructures-and-defects-in-transmi




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